Alginate-containing compositions for use in battery applications

ABSTRACT

A silicon-based anode comprises an alginate-containing binder. The many carboxy groups of alginate bind to a surface of silicon, creating strong, rigid hydrogen bonds that withstand battery cycling. The alginate-containing binder provides good performance to the anode by (1) improving the capacity of the anode in comparison to other commercially-available binders, (2) improving Columbonic efficiency during charging and discharging cycles, and (3) improving stability during charging and discharging cycles.

PRIORITY CLAIM

This application is a Divisional of U.S. patent application Ser. No.13/227,471 filed Sep. 7, 2011, which is a Continuation-in-part of PCTApplication No. PCT/US2011/35072 filed May 3, 2011, which claims thebenefit of U.S. Provisional Patent Application Ser. Nos. 61/330,461 and61/358,465, filed 3 May 2010 and 25 Jun. 2010, respectively, all ofwhich are incorporated herein by reference in their entirety as if fullyset forth below.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The various embodiments relate generally to polymer binders for porouscomposites used in energy storage devices, such as electrodes in primaryand secondary batteries, double-layer capacitors, electrochemicalcapacitors, supercapacitors, electrochemical capacitor-battery hybriddevices, as well as dense (non-porous) composite dielectric layers indielectric capacitors, and polymer separators for use in primary andsecondary batteries, electrochemical capacitors, supercapacitors,double-layer capacitors, and electrochemical capacitor-battery hybriddevices.

2. Description of the Relevant Art

Growing efficient materials, components, and structures from plants areof the highest interest for the sustainable future, due to thepreservation of the environment during the plant growing processes and aplant's ability to efficiently capture carbon dioxide. Particularlyattractive are marine plants, such as algae, that can be grown onnon-agricultural land, such as salt water or waste water, and need onlya fraction of the area required by conventional crops.

Due to rapidly increasing renewable energy demands, energy harvesting byocean plants has drawn interest in recent years. Equally important isthe development of high-performance, eco-efficient components for energystorage devices, such as batteries. Several breakthroughs have recentlybeen achieved in the formation of organic cathodes and anodes forlithium-ion batteries. These bio-derived active materials show greatpromise, however they offer limited stability and capacity properties.

A typical procedure for the preparation of Li-ion battery electrodesincludes mixing electro-active powder with conductive carbon additivesand a polymeric binder dissolved in a solvent. The produced slurry isthen casted on metal foil current collectors and dried. Traditionally,most research has been focused on synthesis of active powders withimproved properties and less attention was devoted to the advancement ofthe electrically inactive components of battery electrodes, such asbinders. Yet, recent studies have shown that many important batterycharacteristics, including stability and irreversible capacity losses,are critically dependent on the binder's properties. High capacityelectrochemically active particles that exhibit significant volumechanges during insertion and extraction of Li require improved bindercharacteristics to ensure electrode integrity during use. Si, inparticular, exhibits the largest volume changes during Li-ion batteryoperation. The interest in Si-based anodes stems from the abundance ofSi in nature, its low cost, and its high theoretical capacity, which isan order of magnitude higher than that of the conventionally usedgraphite.

Recent studies have shown that synthetic and bio-derived polymers whichcontain carboxy groups, such as polyacrylic acid (PAA) and carboxymethylcellulose (CMC), demonstrate promising characteristics as binders forSi-based anodes. Low binder extensibility did not demonstrate a negativeeffect on the battery performance. Reasonably stable anode performance,however, could only be achieved when Si volume changes were minimized byincomplete Li insertion in the tests or accommodated by usingextra-large binder content, which lowers the resulting anode capacity.The polar hydrogen bonds between the carboxy groups of the binder andthe SiO₂ on the Si surface were proposed to exhibit a self-healingeffect and reform if locally broken. An alternative explanation for theobserved stability of the rigid binders with lower extensibility couldbe that Si nanoparticles deform plastically during electrochemicalalloying with Li, expanding towards the existing pores between theparticles.

SUMMARY OF THE INVENTION

Various embodiments of the present invention provide an energy storagedevice, comprising at least one electrode, wherein the at least oneelectrode comprises an alginate-containing composition. In exemplaryembodiments, the alginate-containing composition is a binder. Thealginate-containing composition can form a porous film that binds to atleast a portion of a surface of the at least one electrode.

The alginate-containing composition can be alginate, alginic acid, or asalt of an alginic acid. Further, the salt of the alginic acid can beNa, Li, K, Ca, NH₄, Mg, or Al salt of alginic acid. Thealginate-containing composition can have a molecular weight of about10,000 to about 600,000. In exemplary embodiments, the alginatecontaining composition has a molecular weight of about 200,000. Thealginate-containing composition can be chemically or physicallycross-linked. Further, the alginate-containing composition can compriseanother polymer grafted with, cross-linked with, or blended withalginate. The polymer can be a water soluble polymer, organic solublepolymer, insoluble polymer, or combinations thereof.

In some embodiments, the alginate-containing composition can be about0.5 weight percent to about 60 weight percent of the at least oneelectrode. In other embodiments, the alginate-containing composition canbe about 2 weight percent to about 25 weight percent of the at least oneelectrode. In exemplary embodiments where the alginate-composition formsthe porous film, the porous film can have a thickness of about 1 micronto about 40 microns.

In other exemplary embodiments, the alginate-containing composition canbe a separator. The alginate-containing separator can be formed as acoating on the electrode or formed as a stand-alone separator.

The energy storage device of the various embodiments can be an electricdouble layer capacitor, a supercapacitor, an electrochemical capacitor,a primary battery, a secondary battery, a battery-electrochemicalcapacitor hybrid device, or an electrochemical energy storage device.

Other exemplary embodiments of the present invention provide adielectric capacitor, comprising at least one dielectric layer, whereinthe at least one dielectric layer comprises an alginate-containingcomposition. The alginate-containing composition can be a binder forparticles having a dielectric constant in the range of about 3 to about60,000. The thickness of the at least one dielectric layer can be about0.05 microns to about 50 microns. In exemplary embodiments, thealginate-containing composition can be about 0.8 weight percent to about80 weight percent of the at least one dielectric layer. In otherexemplary embodiments, the alginate containing composition has amolecular weight of about 500 to about 800,000.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will become apparent to thoseskilled in the art with the benefit of the following detaileddescription of embodiments and upon reference to the accompanyingdrawings in which:

FIGS. 1A-C graphically compare viscosities of sodium alginate (Naalginate) and sodium carboxymethyl cellulose (Na CMC) binders as afunction of weight percent (wt. %) in water, as a function of the shearrate, and as a function of temperature;

FIGS. 2A-B graphically illustrate electrochemical performancecharacteristics of a silicon anode including an alginate-containingbinder;

FIG. 3 provides a scanning electron microscopy (SEM) image of siliconnanopowder;

FIGS. 4A-B provide SEM images of silicon nanopowder;

FIG. 5 provides ¹H nuclear magnetic resonance (NMR) data of a Naalginate sample;

FIGS. 6-9 provide atomic microscopy (AFM) data comparing Young's modulusof Na alginate and PVDF binders in both dry and wet states;

FIG. 10 provides SEM images of silicon nanoparticles;

FIG. 11 provides energy dispersive spectroscopy (EDS) and X-raydiffraction data (XRD) of silicon nanoparticles;

FIG. 12 provides N₂ sorption isotherm data collected on siliconnanopowder at 77 K;

FIG. 13 provides an SEM image of silicon nanopowder bonded with a Naalginate binder and forming an electrode;

FIGS. 14 and 15 provide X-ray photoelectron spectroscopy (XPS) data ofthe initially used silicon powder, Na alginate, silicon-Na (Si—Na)alginate, and silicon obtained from the silicon-Na alginate electrodeafter dissolution and multiple washing steps;

FIG. 16 provides Fourier transform infrared (FTIR) spectroscopy data ofNa-alginate, the Si—Na alginate electrode, and the Si powder (used forthe electrode formulation);

FIG. 17 graphically illustrates reversible deintercalation specificcapacity of a silicon anode comprising 15 wt. % Na alginate, 64 wt. % Sinanoparticles, and 21 wt. % C particles (C conductive additives)normalized by the weight of Si and C combined;

FIGS. 18A-C provide XPS data of electrodes before and after cycling inLi half cells within the potential range from 0.01 to 1 V vs. Li/Li⁺;

FIG. 19 graphically illustrate differential capacity curves for lithiuminsertion into and lithium extraction from the Si, C, andalginate-containing composite electrode;

FIGS. 20 and 21 graphically illustrate the shape of the galvanostaticlithium insertion and extraction profiles of the Si, C, andalginate-containing composite electrode;

FIG. 22 graphically compares first charge-discharge profiles of thegraphite and alginate-containing electrode;

FIG. 23A depicts the Young's modulus of Na-CMC in a dry state. FIG. 23Bdepicts the Young's modulus of Na-CMC in a wet (impregnated withelectrolyte solvent) state;

FIG. 24 A depicts the XPS characterization (O_(1s) high resolutionspectra) of alginate, carbon black (CB), CB electrode prepared by mixingcarbon additives with Na alginate binder, and CB powder extracted fromthe CB electrode after extensive purification. FIG. 24B depicts the XPScharacterization (C_(1s) high resolution spectra) of alginate, carbonblack (CB), CB electrode prepared by mixing carbon additives with Naalginate binder, and CB powder extracted from the CB electrode afterextensive purification. FIG. 24C depicts the XPS characterization(O_(1s) high resolution spectra) of purified exfoliated graphite (PEG),alginate, PEG electrode prepared by mixing PEG with Na alginate binder,and PEG powder extracted from the PEG electrode after extensivepurification. FIG. 24D depicts the XPS characterization (C_(1s) highresolution spectra) of PEG powder, alginate, PEG electrode, and PEGpowder extracted from the PEG electrode after extensive purification;and

FIG. 25 depicts the electrochemical performance of alginate-based nanoSielectrodes (electrode density=0.50 g cm-3, weight ratio of Si:C=3:1).

FIG. 26 depicts the electrochemical performance of a Si anode with PVDFand Na-CMC.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but to the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be described hereinin detail. Throughout this description, various components can beidentified as having specific values or parameters, however, these itemsare provided as exemplary embodiments. Indeed, the exemplary embodimentsdo not limit the various aspects and concepts of the present inventionas many comparable parameters, sizes, ranges, and/or values can beimplemented.

It should also be noted that, the singular forms “a,” “an,” and “the”include plural references unless the context clearly dictates otherwise.For example, reference to a component is intended also to includecomposition of a plurality of components. References to a compositioncontaining “a” constituent is intended to include other constituents inaddition to the one named. It is intended that each term contemplatesits broadest meaning as understood by those skilled in the art andincludes all technical equivalents which operate in a similar manner toaccomplish a similar purpose.

Values may be expressed herein as “about” or “approximately” oneparticular value, this is meant to encompass the one particular valueand other values that are relatively close but not exactly equal to theone particular value. By “comprising” or “containing” or “including” ismeant that at least the named compound, element, particle, or methodstep is present in the composition or article or method, but does notexclude the presence of other compounds, materials, particles, methodsteps, even if the other such compounds, material, particles, methodsteps have the same function as what is named. It is also to beunderstood that the mention of one or more method steps does notpreclude the presence of additional method steps or intervening methodsteps between those steps expressly identified. Similarly, it is also tobe understood that the mention of one or more components in acomposition does not preclude the presence of additional components thanthose expressly identified.

The term “nanoparticles” as used herein refers to refers to particlesthat have an average diameter in the range of 500 nanometers to 1nanometer; or from 250 nanometers to 10 nanometers, or from 100nanometers to 20 nanometers.

The various embodiments of the present invention generally relate to theuse of alginate-containing compositions in energy storage devices. Morespecifically, exemplary embodiments of the present invention relate tothe use of alginate-containing compositions as binders for electrodes inenergy storage devices, such as electrical double layer capacitors,electrochemical capacitors, supercapacitors, primary and secondarybatteries and various electrochemical capacitor-battery hybrid devices.More specifically, exemplary embodiments of the present invention relateto the use of alginate-containing compositions as binders forlithium-ion battery electrodes.

Due to millions of years of evolution, the molecular architecture ofalginates have been optimized to facilitate growth and survival of brownalgae and many microorganisms in strong electrolyte environments, whichis desirable for energy storage devices, such as batteries. Brown algae,rich in alginates, is not only one of the fastest growing plants on theplanet, but is also the longest and the heaviest of all the seaweeds,therefore enhancing the accessibility of alginate and, consequently,lowering its cost. Alginate (commonly in a sodium salt form) can beextracted from algae by heating the algae in a hot soda (Na₂CO₃)solution. Alginate, also referred to as alginic acid, is a copolymer of1→4 linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues.Different compositions and sequences of M and G monoblocks in alginatesyield a plethora of physical and biological properties, optimized inbrown algae for a given environment, and thus are subject to change dueto seasonal and growth conditions. For example, algae growing in coastalareas have higher G content than the same algae growing in streamingwaters. A high content of G acid makes alginate gels more rigid.Multivalent ions from seawater can crosslink the matrix, also increasingthe rigidity of the plant body. Different algae species and bacteriaproduce alginates with different G-M composition and monoblock lengths.Additionally, the alginate composition can be altered using enzymaticpost-modification, making it a very versatile substance.

These unique alginate properties also make it desirable for use inenergy storage devices, such as electrical double layer capacitors,supercapacitors, electrochemical capacitors, dielectric capacitors,electrochemical cells, secondary and primary batteries, orelectrochemical energy storage devices. First, unlike manypolysaccharides commonly found in terrestrial plants, alginates uniquelycontain carboxylic groups at each monomeric unit of the polymer. Thishigh content of carboxylic groups is desirable for energy storage devicebecause carboxylic groups readily form rigid, hydrogen bonds withoxidized surfaces, which enhances the stability and mechanical strengthof energy storage device components, for example, electrodes. Second,alginate easily dissolves in water. This solubility characteristiclowers manufacturing costs, simplifies processing steps, and makesalginate more environmentally friendly than other binders.

Alginate-containing compositions can be used in energy storage devices,for example, as a separator, a protective electrode coating, and/or abinder. More specifically, alginate-containing compositions can be usedas binders in cathodes and anodes of batteries, for example lithium-ion(Li-ion) batteries, magnesium-ion (Mg-ion) batteries, aluminum-ion(Al-ion) batteries, or sodium-ion (Na-ion) batteries, to name a few. Thecarboxy groups in alginate bind well to most cathode and anodematerials, (for example to Si or C or various metal oxides) and tocurrent collectors, and provide stability and high Coulombic efficiencyduring charging and discharging cycles. Conventional binders currentlyused for silicon-based and graphite-based anodes, specifically, arecarboxymethylcellulose (“CMC”) (or its salts) and poly(vinylidenefluoride) (“PVDF”). These binders have many limitations. CMC compriseslimited amounts of carboxylic groups capable of binding to the cathodeor anode material. Other disadvantages of CMC are the lack of controlover their configuration (placement of the carboxy groups in themacromolecules) and, more importantly, the need for rigorous chemicalprocedures for their synthesis. CMC synthesis, for example, involves thealkali-catalyzed reaction of cellulose with chloroacetic acid tointroduce carboxy groups, which are responsible for its chemicalreactivity and, in a salt form, solubility in water. Cellulose itself, astructural component for most plants, does not contain carboxy groupsand is practically insoluble.

The PVDF binds to Si (or graphite) particles via relatively weakvan-der-Waals forces and does not accommodate large changes in spacingbetween silicon particles caused by expanding and contracting duringcycling. Thus, the conventional binders can be inefficient in holdingthe silicon particles together and maintaining electrical conductivitywithin the anode, which is important for efficient battery operation.Alginate-containing binders do not present these limitations. Inaddition, they allow active electrode particles to be uniformlydispersed in a slurry and thus allow very uniform electrode fabricationprocedure, which is important for reproducible performance and goodcycle life of the energy storage devices comprising such electrodes.

Silicon may be used as the high-capacity active material for a Li-ionbattery anode. However, Si presents certain technical challenges, forexample, as a Li-ion battery cycles, the electrochemical alloying (anddealloying) of silicon and lithium causes volume changes, particularlyparticle expansion upon lithium insertion into a silicon orsilicon-lithium particle contraction during lithium extraction from asilicon-lithium alloy particle. Such volume changes can compromise theinterface between the silicon and its binder, known as thesolid-electrolyte interphase (“SEI”). For example, a portion of thebinder may lift off the silicon, thus creating a void in the interfacewhich allows electrolyte solvent remaining in the anode to possiblycreep into the remaining portion of the interface, breaking the bondingbetween the binder and the silicon, and destroying the interface. If theinterface is damaged, the interface may not be strong enough to bemaintained when the silicon swells. Further, when an electrolyte solvententers the SEI and reaches Si, electrolyte decomposes which contributesto the growth of the SEI and loss of Li, which, in turn decreases theoverall cycle life of the anode. Alginate-containing binders provide therigidity and chemical properties necessary for desirable batteryoperation.

In exemplary embodiments, alginate-containing compositions are used asbinders in silicon-based anodes for Li-ion batteries. The source of thesilicon in the silicon-based anodes includes silicon particles in theform of spheres, agglomerates, needles, rods, fibers, nanotubes andvarious other forms of silicon. The source can also be silicon-carboncomposite materials, wherein such composite refers to mixing of siliconparticles with carbon, coating silicon particles with carbon, coatingcarbon particles with silicon, creating three-dimensional dendriticparticle structures by coating carbon with silicon and again withcarbon, and other variations involving chemical and physicalcombinations of silicon and carbon. In exemplary embodiments, thesilicon-carbon composite can contain about 50 weight percent to about 95weight percent silicon and about 5 weight percent to about 50 weightpercent carbon. The source of silicon can also be silicon alloys, suchas alloys with X (where X is N, Ge, Be, Ag, Al, Mg, Cd, Ga, In, Sb, Sn,and Zn), that are capable of forming a better solid-electrolyteinterphase (SEI) on the Si—X surface than pure silicon. It should beunderstood that while the disclosure herein primarily referencessilicon-based anodes for Li-ion batteries, other electrode (inclusive ofcathodes and anodes) materials, such as other materials thatelectrochemically alloy with lithium (such as Sn, Ge, Mg, Al, andothers), as well as graphite, graphene, various graphitic and disorderedcarbons, carbon-containing composite materials, electrochemically activepolymers capable of storing ions, phosphorous, tin oxide, iron, ironoxide, zinc, manganese, nickel, sulfates, titanium oxide, sulfur (S),carbon-sulfur composites, various other sulfur-containing composites,various transition metal nitrides, transition metal oxides (such as ironoxides, vanadium oxides, molybdenum oxides, among others), transitionmetal sulphides (such as TiS, TiS₃, MoS₂, among others), various lithiumcontaining metal oxides (such as LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄,LiFePO₄, LiFeVO₄, LiMn_(x)Co_(y)O_(z), various other oxides with thegeneral formula of Li_(x)M_(y)N_(z)OF_(a), where M is selected from afirst group consisting of Ni, Mn, V, and Co; N is selected from a secondgroup consisting of transition metals and phosphorus; M and N arenon-identical; Li is lithium, O is oxygen, and F is fluorine; andwherein subscripts x, y, z, and a are non-zero or zero), lithiumsilicates, and various other ion-hosting materials can be used in placeof Si for electrodes in primary and rechargeable Li-ion batteries.Furthermore, materials other than Si can be used withalginate-containing compositions for Mg-ion, Al-ion, Na-ion and othermetal-ion batteries, other types of primary and rechargeable batteries,electrical double layer capacitors, electrochemical capacitors,supercapacitors, and various electrochemical capacitor-battery hybriddevices. One skilled in the art will appreciate that the examplesdescribed herein are not solely limited to silicon-based anodes forLi-ion batteries and can also apply to anodes for electrochemical energystorage devices having other active materials and to cathodes forelectrochemical energy storage devices having different activematerials. Finally, alginate-containing compositions can be used asbinders for ceramic particles for use in a dielectric layer inpolymer-ceramic composite dielectric capacitors.

It should be understood that in the electrodes containing alginate andactive (anode or cathode) materials other components may be added aswell. These include various conductive additives (such as carbon, metalsand conductive polymers) as well as other functional materials that areadded to improve the battery cycle life or power characteristics orsafety or other useful functions.

It should be understood that while the disclosure herein primarilyreferences sodium salt of alginic acid (Na-alginate), other types ofalginate, such as alginic acid and various other salts of alginic acid(such as potassium (K) alginate, calcium (Ca) alginate, ammonium (NH₄)alginate, lithium (Li) alginate, aluminum (Al) alginate, magnesium (Mg)alginate, among other salts of alginic acid) can be used as well.

The lithium salt of alginate may be preferred for lithium ion batteries.The first cycle irreversible capacity losses of Si-containing electrodesmay often range from 200 to 600 mAh/g, which is too high for manyapplications and highly undesirable. The inventors have discovered thatthe use of Li salt of alginate reduces such irreversible capacity lossesduring the first cycle and often provides better cycle stability of theanodes, including Si-containing anodes in particular. Exemplaryelectrolytes are based on Li salts electrochemically stable in theelectrochemical window and temperature range from −30 C to 70 C, such asLiPF₆, LiN(CF₃SO₂)₂ and their combinations, to name a few. It ispreferred for the electrode to have a powdered active material capableof undergoing lithiation and delithiation and a binder containing 10 to100 weight percent of lithium salt of alginate. The active materialpreferably contains 50 to 100 weight percent Si and 0 to 50 weightpercent C and the amount of the binder ranges from 5 to 20 weightpercent of the total electrode composition.

It should be further understood that while the disclosure hereinprimarily references alginate, other natural or synthetic polymers,mixed, blended or cross-linked with alginate can be used as well.Examples of suitable polymers include but are not limited to celluloseethers, or guar, or their derivatives or polymers composed of diene andother unsaturated (vinyl) monomers or their salts. Such diene monomersinclude 1,3 butadiene, isoprene, chloroprene, cyclobutadiene and divinylbenzene. Unsaturated monomers include alkyl acrylates, hydroxylatedalkyl methacrylates, alkyl vinyl ketones, substituted acrylamides,methacrylic acid, N-methylol acrylamide, 2-hydroxyethyl acrylate, cronicacid, itaconic acid, fumaric acid, maleic acid, maleic anhydride, vinylhalides, vinylidene halides, vinyl esters, vinyl ethers, vinylcarbazole, N-vinyl pyrrolidone, vinyl pyridine, chlorostyrene, alkylstyrene, ethylene, propylene, isobutylene, vinyl triethoxy silane, vinyldiethylmethyl silane, vinyl methyl dichloro silane, vinyl diethylmethylsilane, vinyl methyl dichloro silane, triphenyl vinyl silane, methylmethacrylate, vinyl acetate, acrylonitrile, acrylic acid, acrylamide,maleic anhydride, monovinyl silicon compounds, ethyl vinyl ether,chlorostyrene, vinyl pyridine, butyl vinyl ether, 2-ethylhexyl acrylate,isoprene, chloroprene, vinylidene chloride, butyl vinyl ether andstyrene, to name a few. Suitable cellulose ethers, guars and theirderivatives include but are not limited to carboxymethyl cellulose(CMC), hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose,ethyl hydroxyethyl cellulose, methylcellulose, hydrroxpropyl cellulose,methylhydroxyethyl cellulose, ethyl guar, methylhydroxypropyl cellulose,cationic guar, carboxymethyl guar, hydroxypropyl guar, carboxymethylhydroxypropyl guar, to name a few. Other polysaccharides and theirderivatives can be utilized as well. Examples include, but not limitedto dextran, pectins, starch, heparin, fucoidan and various sulfonatedforms of polysaccharides.

Desirable performance characteristics of silicon-based anodes includehigh specific capacity, high Coulombic efficiency and long cycle lifeduring charging and discharging cycles. Accordingly, alginate-containingbinders provide good performance to the anode by (1) improving thecapacity of the anode in comparison to other commercially-availablebinders, (2) improving Coulombic efficiency during charging anddischarging cycles, and (3) improving stability during charging anddischarging cycles.

The carboxy groups of the various embodiments of the alginate-containingbinder described herein interact strongly with SiO₂ (present on siliconparticle surfaces) via rigid, polar hydrogen bonding. This bondingexhibits self-healing effects and reform when locally broken, thusenabling silicon-based anodes to withstand silicon cycles of expansionand contraction during charging and discharging cycles. Thesecharacteristics are desirable, particularly for silicon-based anodes,because silicon exhibits large volume changes during insertion andextraction of lithium during lithium-ion battery operation. These largevolume changes cause silicon-based anodes to degrade quickly. The use ofsilicon-based anodes, however, remains desirable because of silicon'sabundance in nature, low cost and high theoretical capacity, an order ofmagnitude higher than that of traditionally used graphite. Stability andirreversible capacity losses of silicon-based anodes, specifically arelinked to the binders' properties, thus the strong and rigid bindingbetween the carboxy groups in alginate-containing binders and siliconparticles slows the overall degradation rate and improves the stabilityof silicon-based anodes.

As described above, volume changes can compromise the SEI, which causesa portion of the binder to lift off the silicon, which creates voids inthe interface which allows solvent remaining in the anode to creep intothe remaining portion of the interface and break the bonding between thebinder and the silicon, thus destroying the interface. A protective filmis therefore desirable to prevent continuous access of an electrolytesolvent onto the surface of active silicon particles, which leads toelectrolyte decomposition, degradation of the SEI layer and eventualelectrical separation of active Si—Li particles upon their expansion andcontraction at each charge-discharge cycle. Alginate-containing bindersprovide this protective film as they do not interact with conventionalelectrolyte solvents, thus if the SEI is compromised during expansioncycles, the alginate-containing binder can shield the silicon particlesfrom electrolyte solvent that may compromise the performance of thesilicon-based anode.

As described above, the alginate-containing binder can be, for example,alginic acid, a salt of aliginic acid (also called alginate salt).Alginate can be extracted from algae by heating algae in a hot soda(Na₂CO₃) solution, thus the alginate-containing binder is in a sodiumsalt form in exemplary embodiments. Further, because alginate is veryeasy to cross-link, the alginate-containing binder can be in eithercross-linked or non-cross-linked form. Additionally, thealginate-containing binder can also comprise alginate salt cross-linked,blended with, or grafted with a polymer. The polymer can be, forexample, other commercially available binders, such as CMC or otherwater-soluble polymers.

The alginate-containing binder demonstrates shear-thinning behavior. Oneskilled in the art will appreciate that the viscosity of analginate-containing composition depends on how fast it is mixed (i.e.,dissolved) in water. An increase in the mixing rate consequentlydecreases the viscosity of the alginate-containing composition andprovides a more uniform distribution of the particles in the slurry usedto prepare the electrode material. Conversely, if thealginate-containing composition is dissolved in water without mixing,the viscosity is increased, which effectively stabilizes the slurry fromsedimentation. Further, the alginate-containing composition can comprisealginate compositions of many molecular weights. One skilled in the artwill appreciate that an increase in molecular weight consequentlyincreases the viscosity of the alginate-composition and, conversely, adecrease in molecular weight consequently decreases the viscosity. Inexemplary embodiments the molecular weight of the alginate compositionused for the binder can range from approximately 10,000 to 600,000,preferably 200,000, however the binder is not limited to these molecularweights. As an example, a comparison of the viscosities of sodiumalginate and sodium CMC in water as a function of its weight percent inwater are illustrated in FIGS. 1A and B. More specifically, FIGS. 1A and1B show the viscosity of 1% sodium alginate and CMC solutions in water.Alginate solution viscosity is much higher relatively to CMC, which iscritical for uniform slurry formation. CMC viscosity is very low (i.e.,less than 50 cP, and cannot be accurately measured with equipmentcommonly used). To get a viscosity comparable to that of alginate, oneneeds significantly higher content of CMC in water. This high content ofthe binder is undesirable because it decreases the electrode electricalconductivity, increases its weight and may impede the ion transportthrough the electrode. If needed, the viscosity of thealginate-containing slurry can be decreased by increasing the slurrytemperature (FIG. 1C). By varying viscosity one may optimize the slurryfor the preparation of electrodes with the desired porosity levels. Someelectrodes (such as Si) require sufficient pore volume available for theexpansion during insertion of Li. Other electrodes, require only verysmall open pore volume. Therefore, capability to vary the slurryviscosity via changing the temperature and/or shear rate is critical foroptimization of the electrode preparation.

Further, the alginate-containing binder can have many puritycharacteristics. For example, algae can be grown in waste water or saltwater. One skilled in the art will appreciate that algae grown insaltwater is more pure than algae grown in waste water. However,alginate compositions extracted from both waste water and salt water canbe used as a binder. One skilled in the art will also appreciate thatwaste water algae is less expensive, which consequently reduces costsassociated with manufacturing. The versatility of alginate allowssilicon-based anodes and other battery components to be customized toachieve desired parameters.

In exemplary embodiments, the alginate-containing binder can be about0.5 weight percent to about 30 weight percent of the anode. In exemplarysilicon-anode embodiments, the alginate-containing binder can be about15 weight percent of the anode. In exemplary graphite-anode embodiments,the alginate-containing binder can be about 5 weight percent of theanode. In other exemplary embodiments, the silicon can be coated with acarbon coating to improve the electrical conductivity within the anode,improve the properties of the SEI, enhances temperature stability, andreduce degradation of the electrolytes, and the alginate-containingbinder can be about 5 to 20 weight percent of the anode.

Further, other exemplary embodiments can also comprise a carbonate-basedadditive, such as vinylene carbonate (VC) or fluoroethylene carbonate(FEC) or combinations thereof, which helps seal the interface betweenthe silicon and the electrolyte and improves stability of the SEI layer,so that these interfaces are not compromised during operation. Thecarbonate-based additive can be formulated into the electrolyte or intoa carbonate-containing material operative to have time-dependent releaserates of the carbonate during battery operation, battery storage, orduring “formation cycles” performed by a battery manufacturer. Thegradual rate of vinylene carbonate release can range, for example, fromone day to three hundred days. The carbonate-releasing material can bein the form of particles added into the anode, cathode, or electrolyte,or can be a part of the membrane separating the cathode from the anode.This carbonate-containing material can be incorporated into thealginate-containing composition as a part of the alginate-containingbinder or an alginate-containing separator or both.

In exemplary embodiments, the alginate-containing binder is combinedwith carbon-coated Si-containing composite particles with Si content inthe range of 50 to 95 weight percent and surface area in the range from2 to 100 m²/g, and an electrolyte containing either FEC or VC additivesor both in the amount ranging from 3 to 15 total weight percent. Inanother exemplary embodiment, the alginate-containing binder is combinedwith carbon-coated Si-containing composite particles with Si content inthe range of 50 to 95 weight percent and surface area in the range from2 to 100 m²/g, and an electrolyte containing either FEC or VC additivesor both in the amount from 3 to 15 total weight percent and containingpropylene carbonate (PC) solvent in the amount ranging from 15 to 97weight percent.

In a further embodiment, an imide salt is added to the electrolytecontaining a solvent to improve the stability of the SEI and Siparticles, to improve high temperature performance of theelectrochemical cell with an alginate-containing electrode, to suppressthe reaction between the organic solvent and the electrode material dueto the presence of imide compound(s) that complex with the lithium ions,and the cycle characteristics of the Li-ion battery is improved.Preferable choices for the imide salts include lithiumbis(trifluoromethane)sulfonamide, lithiumbis(pentafluoroethanesulfonyl)imide, lithiumcyclo-difluoromethanebis(sulfonyl)imide, lithium N-hydroxyphthalimide,lithium N-hydroxysuccinimide, lithium N,N-disuccinimidyl carbonate,lithium 1,5-bis(succinimidoxycarbonyloxy)pentane, lithium9-fluorenylmethyl-N-succinimidyl carbonate, lithiumN-(benzyloxycarbonyloxy)succinimide, and lithiumZ-glycine-N-succinimidyl ester.

In an exemplary embodiment, the alginate-containing binder is combinedwith carbon-coated Si-containing composite particles with Si content inthe range from 50 to 95 weight percent and surface area in the rangefrom 2 to 100 m²/g, an electrolyte containing either FEC or VC additivesor both in the range from 3 to 15 total weight percent, and the imidesalt of lithium bis(trifluoromethanesulfonyl) in the concentrationranging from 0.1 to 1.5 M. In another exemplary embodiment, thealginate-containing binder is combined with carbon-coated Si-containingcomposite particles, having Si content in the range from 50 to 95 weightpercent and surface area in the range from 2 to 100 m²/g, an electrolytecontaining either FEC or VC additives or both in the amount ranging from3 to 15 total weight percent, a propylene carbonate (PC) solvent in theamount ranging from 15 to 97 percent, and an imide salt of lithiumbis(trifluoromethanesulfonyl) imide in the concentration ranging from0.1 to 1.5 M. PC in the discussed embodiment was discovered to improvethe kinetics of the electrochemical insertion/extraction of Li ionsinto/from Si-containing electrodes while providing improved SEIstability and better overall performance of the Li-ion cells withSi-containing electrodes.

The various embodiments of the silicon-based anode withalginate-containing binder can be manufactured in many ways. Siliconparticles can be suspended in a solvent to create a suspension. In someexamples the silicon particles can be silicon alloy particles with highsilicon content, for example, silicon-germanium alloy particles,silicon-tin alloy particles, or silicon-germanium-tin alloy particles,with the atomic percentage of silicon in the silicon-germanium,silicon-tin or silicon-germanium-tin alloys ranging from 50 to 99.999%,preferably higher than 70%. In some examples the Si particles can beincorporated in a matrix comprising carbon or other materials.

In exemplary embodiments, the silicon is suspended in solvent atapproximately 10% weight/volume of silicon weight to solvent volume. Thesolvent can be, for example but not limited to, methanol, ethanol,water, or any combinations thereof. In exemplary embodiments, thesuspension is sonicated for approximately 60 minutes. In someembodiments, a carbon coating, for example but not limited to, a carboncoating produced by pyrolysis of polycarbonate, propylene, acetylene, ormethane, is used to coat the surface of the silicon particles. In thevarious embodiments, the silicon particles can be as small as 10nanometers (nm) or in excess of 30 microns. Even more specifically, thesilicon particles can range between 100 and 800 nm. In embodimentswherein the silicon particles are coated with the carbon coating, thesuspension can comprise 5 to 30 weight percent of carbon coating tosilicon. The thickness of the coating typically depends on the viscosityof the suspension and the size of the silicon particles, but may beaffected by other factors as well. Commonly, the thickness is selectedin such a way as to provide the desired anode capacity per unit area tomatch with the capacity of the cathode. In some examples, conductivecarbon additives are added together with silicon particles into thesuspension to improve the electrical conductivity of the anode. Thealginate-containing composition can then be added to the suspension. Thesuspension can then be stirred and subsequently sonicated. This enablesthe carboxy groups of the alginate-containing composition to bind to atleast a portion of the oxidized silicon particle surface via stronghydrogen bonds. Vinylene carbonate (VC) or FEC or both or a materialcontaining and capable of releasing VC or FEC or both if used, can thenbe added to the suspension.

In another exemplary embodiment, the alginate-containing electrode isprepared from a slurry composed of water, an alginate-containing binder,Si-containing particles, and optional conductive carbon additives withthe solid-to-water weight ratio ranging from 1:3 to 1:40 and withalginate-to-particle weight ratio ranging from 1:20 to 1:1.5. TheSi-containing particles can be various forms of silicon, silicon coatedwith carbon, carbon coated with silicon, silicon-carbon dendriticparticles, or combinations thereof. The alginate-containing binder canbe sodium alginate or lithium alginate, or combination thereof. Thepreferred preparation of such a slurry is dissolution of alginate inwater under conditions needed to achieve the viscosity ranging from 10cP to 20,000 cP, preferably in the range from 100 cP to 7000 cP, withaddition of Si-containing particles and optional conductive carbonadditives into this solution and mixing for 0.5 to 24 hours. This slurryis coated on a current collector and the coated current collector isdried at 80° C. to 160° C.

According to some embodiments of the present invention, an anodeincludes a porous composite comprising a plurality of agglomeratednanocomposites. At least one, and as many as all, of the plurality ofnanocomposites includes a dendritic particle formed from athree-dimensional, randomly-ordered assembly of nanoparticles of anelectrically conducting material and a plurality of discrete non-porousnanoparticles of a non-carbon Group 4A element or mixture thereof (i.e.,silicon, germanium, tin, lead, and an alloy or solid solution thereof)disposed on a surface of the dendritic particle. At least onenanocomposite of the plurality of agglomerated nanocomposites has atleast a portion of its dendritic particle in electrical communicationwith at least a portion of a dendritic particle of an adjacentnanocomposite in the plurality of agglomerated nanocomposites. In somecases, the electrically conducting material of the dendritic particlecan be amorphous or graphitic carbon. For example, the amorphous carboncan be carbon black. The non-carbon Group 4A element or mixture thereofis silicon. In certain situations, the porous composite also includes anelectrically conducting coating disposed on at least a portion of asurface of a dendritic particle of at least one of the plurality ofagglomerated nanocomposites. The electrically conducting coating can beformed from carbon, too.

In an embodiment, dendritic particles may be formed from a plurality ofdiscrete nanoparticles of the electrically conducting material.Dendritic particles may be formed by using a thermal treatment (e.g.,sintering or annealing the particles together), sonication, chemicallyreacting the nanoparticles with one another, spontaneously (e.g., via areduction in the surface energy of adjacent nanoparticles), and/or thelike. In some embodiments, silicon nanoparticles may be disposed on theformed dendritic particles. In some embodiments, the siliconnanoparticles may be grown directly on the surface of the dendriticparticle. Many deposition techniques can be used to do this, including,without limitation, physical vapor deposition and all of the variantsthereof, chemical vapor deposition and all of the variants thereof,sputtering and all of the variants thereof, ablation deposition and allof the variants thereof, molecular beam epitaxy and all of the variantsthereof, electro spray ionization and all of the variants thereof, andthe like. In other embodiments, the silicon nanoparticles can beprepared independently, and then coupled to the surface of the dendriticparticle using physical or chemical means. The coated or uncoateddendritic particles may be incorporated into an electrode of a lithiumion battery using an alginate binder.

It has been discovered that other natural or synthetic polymers, mixed,blended or cross-linked with alginate can be used to tune the viscosityof the alginate-containing electrode slurry. This is useful to tune theresulting porosity of the electrode for a fixed content of a binder anda solvent in order to achieve the most preferred performance inbatteries. The solubility of the various types of natural or syntheticpolymers results in different methods for creating thealginate-containing electrode slurry, which has a preferred totalcontent of binder for Si-based anodes in the range from 7 to 20 wt. %.For water soluble natural and synthetic polymer additions, such aspolyacrylic acid or carboxymethyl cellulose, a polymer slurry and analginate slurry can be individually prepared and then mixed togetherbefore the slurry is coated onto a metal substrate. The water solublenatural or synthetic polymer additions can alternatively be mixed withthe alginate in a high-speed mixer to create a dry blend and then theslurry prepared. For natural or synthetic polymers that are soluble in asolvent, such as polyethylene, polypropylene, ethylene-propylene-dieneterpolymer (EPDM), and sulfonated EPDM, the alginate and polymerslurries can be prepared separately in two different solvents (water forthe alginate and a water/alcohol solvent for the second polymer), andthen combined. Alternatively, the organic soluble polymer can be addedin the form of particles or fibers to the alginate slurry. For naturalor synthetic polymers that are insoluble, such as cellulose nanofibers,an alginate-containing slurry can be prepared and the insoluble polymeris added as an insoluble additive.

In exemplary embodiments, electrodes composed of a mixed binder ofpolyacrylic acid (PAA) and alginate with Si-containing particles can beprepared by preparing two separate slurries, one an alginate slurry andthe other a PAA slurry, and then combining these slurries before coatinga metal substrate to create an electrode. The alginate slurry iscomposed of water, alginate, Si-containing particles and optionalconductive carbon additives with the solid-to-water weight ratio rangingfrom 1:3 to 1:40 and with alginate-to-(Si containing particles) weightratio ranging from 1:20 to 1:1.5. The preferred preparation of thealginate slurry includes dissolution of the alginate in water underconditions needed to achieve the viscosity ranging from 10 to 20,000 cP,the addition of Si-containing particles and optional conductive carbonadditives into this solution and mixing for 0.5 to 24 hours. Theseparate PAA slurry is composed of water, alcohol (preferably ethanol),PAA (preferably having molecular weight in the range from 50,000 to1,000,000), Si-containing particles, and optional conductive carbonadditives with the solid-to-water weight ratio ranging from 1:2 to 1:15,the PAA-to-(Si containing particles) weight ratio ranging from 1:20 to1:1.5, and the alcohol-to-water ratio ranging from 1:1000 to 1:5. Thepreferred preparation of the PAA slurry can include either (i) mixingthe powders and optional conductive additives in a water-alcohol mixture(preferably ethanol) for up to 1 hour with the subsequent addition ofPAA and further mixing for at least mixing for 0.5 to 24 hours, or (ii)a nearly complete dissolution of PAA in a water-alcohol mixture underconditions needed to achieve the viscosity ranging from 1 to 1,000 cPfollowed by the addition of Si-containing particles and optionalconductive carbon additives to this solution and mixing for 0.5 to 24hours. The alginate slurry and the PAA slurry are mixed together withthe preferred weight ratio of PAA-to-alginate ranging from 5:1 to 1:5.

It has been discovered that alginate-containing compositions can alsoprovide benefits to cathodes for Li-ion batteries as well as otherprimary and secondary batteries and electrochemical capacitors. Manycathode materials experience volume changes during insertion andextraction of ions, which may eventually lead to increase in theelectrode resistance and to the mechanical degradation of the electrode,which will decrease the cycle lifetime of the batteries. Therefore, thecapability of alginates to provide strong bonding between cathodeparticles, between cathode particles and conductive additives, andbetween cathode and the current collector, may be beneficial for thelong cycle life of the batteries. Furthermore, in contrast to PVDF,which requires the use of expensive and environmentally-unfriendlyorganic solvents, alginates can use environmentally-friendly solvents(such as water), which will reduce the cost of the cathode and the fullcell fabrication. In addition, many cathode materials experiencedegradations due to the side reactions with electrolyte. For example,hydrofluoric acid present in electrolyte may interact and dissolve thesurface of many oxide-based cathodes. Many cathode materials experiencepartial degradation or partially dissolution of their surface layerduring the battery operation. A coating of alginate-based binders on thesurface of the cathode particle surface may protect these cathodeparticles, mitigate their degradation, and partially neutralize theharmful components of electrolyte. Finally, in order to achieve highquality of the electrodes (either cathodes or anodes), it is critical tohave very high level of uniformity in both the slurry and the electrodesthemselves. The ability of alginates to disperse cathode particles andform stable suspension within the slurry will be beneficial forachieving superior uniformity in the electrodes and thus more consistentperformance and longer lifetime of the batteries. Overall,alginate-containing compositions will be beneficial for improvingdispersion, adhesion strength, electrical connectivity and cycle life inboth cathodes and anodes in various electrochemical energy storagedevices.

It has been discovered that due to excellent mechanical properties ofalginates, the alginate-containing composition may be used as a porouscoating (from 1 to 40 micron in thickness; porosity level from 20 to 95volume %) on the surface of an electrode for various energy storagedevices, including Li-ion batteries. This alginate-containing porousfilm (containing from 1 to 100 wt. % alginate) has several usefulfunctions. First, it strengthens the electrode. Second, it protects theelectrode surface from mechanical damage. Third, it mitigates apossibility of a short circuit. Fourth, it may provide a function of aseparator membrane. In this case, no additional separator membrane isneeded, which reduces the cost of the battery fabrication.Conventionally, porous separator membranes have a thickness of 25 to 60micron and contain thick pore walls in order to provide the neededmechanical support to the membrane. This minimizes the amount of smallpores and increases the overall weight and the volume of the membrane.Small pores, however, are desired to minimize the possibility of thetransport of conductive nanoparticles from one electrode to another,which may lead to short-circuit. Since the proposed alginate-containingporous coating is deposited on the surface of an electrode, it can beproduced thinner than traditional separator membrane and have smallerpores. This will reduce the battery volume and weight, while increasingits power and reliability. Furthermore, functional groups in alginate(alginic acid or its salts) may favorably interact and neutralize theundesired components present in electrolyte, thus improving the overalllife and safety of the batteries. This gives the fifth useful functionto the porous alginate-containing polymer coating.

A separator placed between a cathode and an anode is one of criticalcomponents in the rechargeable lithium batteries. Its primary functionis to effectively transport ionic charge carriers between the twoelectrodes as an efficient ionic conductor as well as to prevent theelectric contact between them as a good electric insulator. A separatorshould be chemically and electrochemically stable and have mechanicalstrength sufficiently enough to sustain battery-assembly processes. Inaddition, it is often desirable for such separators to be wetted by apolar electrolyte solvent. Alginate-based separators were discovered towork particularly well. It was discovered that a porousalginate-containing coating of at least one electrode can function as aseparator in an electrochemical cell composed of positive and negativeelectrodes. It was also discovered that a porous alginate-containingseparator may be a stand-alone separator.

In exemplary embodiments, a porous alginate-containing separator coatingcan be prepared on an electrode by mixing an alginate-containing watersolution with either (i) another polymer or (ii) polymeric(nano)particles or (iii) inorganic (nano)particles or (iiii) combinationof (i-iii) to create a coating mixture and deposited the coating mixtureonto an anode. In one example, after drying the coating mixture onto ananode, the (i) added polymer or the (ii or iii) nanoparticles can bedissolved with a selective solvent, leaving alginate intact, yet porous.The polymer added to the alginate-containing coating mixture should besoluble in solvent other than water to provide selective dissolution ofthe polymer while not of the alginate. Examples of such polymers includebut are not limited to polyacrylic acid, polymethacrylic acid,polyethylene glycol, polyethelene imine, and polyallyl amine. In anotherexample, the (ii or iii) nanoparticles are electrically insulating andcan be left in the alginate-containing porous separator coating. Theresidual alginate coating on top of the electrode will be porous due toeither dissolution of the part of the alginate coating or evaporation ofwater. In another example, the degree of porosity and the pore size ofthe separator coating can be tuned by varying amount of polymer andnanoparticles added; the size of the nanoparticles; and the amount ofwater in the initial coating slurry. In the case of nanoparticles addedto the alginate solution to provide porosity, the nanoparticles can beeither dissolved after coating formation or left in the coating. Polymerparticles which can be dissolved most conveniently (but not limited to)can be prepared by emulsion polymerization from a variety of monomers.Examples include styrene, methacrylates, and acrylates. The synthesis ofsuch nanoparticles is done in the presence of anionic surfactant todisperse the nanoparticles in the water and is necessary to preventparticle flocculation when mixed with alginate solution. Sodium dodecylsulfate (SDS) is an example of an anionic surfactant that can be addedfor nanoparticle synthesis. Emulsion polymerization prepared polymernanoparticles are added to the alginate-containing water solution beforecoating the mixture onto the anode. In the case of nanoparticles addedto the alginate solution to achieve a desired pore size in thealginate-containing separator, the pore size strongly depends on thenature of the added second component (i, ii, or iii) of thealginate-containing separator coating. Addition of polymer may providethe smallest size of the pores. The sizes of the pores created can be assmall as radius of gyration of the polymer molecules added and rangefrom 2 to 20 nm. Larger pore size in the alginate-containing separatorcoating is created by addition of the polymeric (nano)particles or theinorganic (nano)particles and can range from 15 to 10,000 nm indiameter. Inorganic insulating nanoparticles (such as silicates, silica,alumina, zirconia and others) can also be added to the alginate toprovide porosity. In the case of inorganic nanoparticles, suchnanoparticles can be left in the coating intact. The resultingalginate-nanoparticles composite coating has an intrinsic porosity andcan serve as porous inert coating eliminating necessity to add astand-alone separator to the battery.

It has been discovered that it may be desirable that thealginate-containing separator coating does not fill the pores alreadyexisting in the actual electrode. To create the porous alginateseparator only on top of the electrode, the pores in the electrode canfirst be filled with a sacrificial polymer, preferably of low molecularweight and/or non-water soluble, before depositing the alginate coatingonly on the top of the electrode. In an exemplary embodiment, a porouselectrode is first filled with a sacrificial polymer which will preventinsertion of the alginate-containing composition into the electrodepores. This sacrificial polymer is then dissolved with a selectivesolvent that leaves the alginate separator coating intact on the top ofthe electrode. This sacrificial polymer can also be the pore-formingpolymer in the alginate-containing coating mixture that creates theporous alginate-containing separator.

A porous alginate-containing separator coating on top of the anode canbe created from an alginate water solution containing another polymer,such as polyethylene glycol (PEG), added to the alginate solution. Themolecular weight of the PEG can be in the range from 2,000 to 1,000,000Da. The ratio of alginate to PEG can range from 10:1 to 1:1. Thealginate and PEG mixture is deposited onto a preformed anode via bladecoating, spraying, roll coating, or other film-creating method. Aftercoating drying PEG is selectively dissolved. Ethanol, methanol, acetone,THF and other solvents can be used for PEG dissolution.

A porous alginate-containing separator coating on top of the anode canbe created from an alginate water solution containing polymericnanoparticles applied to the anode to perform a separator function. Tocreate the desired porosity of the separator coating, electricallyisolative soluble particles are added to the alginate solution.Polystyrene latex (PS) with particle size 50 to 500 nm diameter ispreferred. PS latex with a negative stabilizing group, such as sodiumdodecyl sulfate, can be utilized. Preferable particle to alginate ratioranges from 1:10 to 1:1. The separator slurry can be deposited via bladecoating, spraying, roll coating and other film-creating methods. Afterslurry deposition and drying of the layer the PS particles are dissolvedby treating the anode with a PS-dissolving solvent, such as methyl ethylketone (MEK). PS is soluble in MEK and will be removed from the coatingforming porous film on top of the anode. The degree of porosity is tunedby varying the amount and size of the PS latex used inalginate-containing separator coating preparation.

A porous alginate-containing separator coating on top of the anode canbe created from an alginate water solution containing inert particles,such as silica particles. The preferable size of the particles is 15 to500 nm in diameter with particle-to-alginate ratio from 1:10 to 1:1. Theseparator slurry can be deposited via blade coating, spraying, rollcoating, or other film-creating method.

In exemplary embodiments, a porous alginate-containing stand-aloneseparator having a thickness of less than 30 micron, porosity in therange from about 25 to about 85%, and composed of alginate can beprepared by several methods including but not limited to one of thefollowing methods. (i) Electrospinning alginate solutions to formnonwoven mats composed of alginate-containing fibers or nanofibers. (ii)Casting a polymer film from a mixture composed of a water solution ofalginate and other polymer or polymeric (nano)particles, followed bydrying and a selective dissolution of the other polymer in a solventthat does not dissolve alginate. The polymer added to thealginate-containing film should be soluble in solvent other than waterto provide selective dissolution of the polymer while not of thealginate. Examples of such polymers include but not limited topolyacrylic acid, polymethacrylic acid, polyethylene glycol,polyethelene imine, and polyallyl amine. (iii) Extruding analginate-containing composition into alginate non-solvent bath, drying,annealing, and stretching to create the porosity and relaxation toreduce internal stresses. (iv) Extruding an alginate-containingcomposition, annealing, stretching to create the porosity and relaxationto reduce internal stresses. (v) Extruding an alginate-containingcomposition into alginate non-solvent bath, drying, annealing, andstretching to create the porosity and relaxation to reduce internalstresses. (vi) Mixing an alginate-containing solution with optionaladditives; extrusion of the heated solution into a sheet; extraction orevaporation of the solvent to form the porous structure. (vii)Preparation of alginate-containing fibers or/and nanofibers;continuously depositing them (or a combination of fibers and nanofibers)on a filter (metal mesh) in a way similar to the preparation of a paperand drying. (viii) Electrospinning of alginate solutions to formnonwoven mats composed of alginate-containing fibers or nanofibers ontothe surface of the preformed anode (as collecting electrode for alginateelectrospinning products).

It has been discovered that an alginate-containing composition can alsobe efficiently used as a binder and a filler in dense polymer-ceramiccomposite dielectric capacitors. A polymer for a polymer-ceramiccomposite material for use in dielectric capacitors should ideallyprovide good mechanical properties, possess high breakdown strength andstrong bonding to the ceramic dielectric particles so that the breakdowndoes not occur at the interface between a ceramic and a polymer.Alginate forms very strong bonding to various ceramic particles andexhibits excellent mechanical properties. Alginate can also form strongbonding to a metal current collector, providing excellent mechanicalintegrity to the metal-dielectric layer-metal capacitor.

EXAMPLES

The various embodiments of the present invention are illustrated by thefollowing non-limiting examples.

Materials and Methods

Materials. Sodium alginate (sodium salt of alginic acid) was derivedfrom Macrocystis pyrifera algae (also called Giant Kelp) and acquiredfrom MP Biomedicals LLC, USA. Si nanopowder (NP—Si-L50, 98% purity, MTICorporation, USA) and C additives (PureBlack® 205-110 and ABG1010, mixedin a 1:1 wt. ratio, all produced by Superior Graphite, USA) were used asactive materials for the Si electrode preparation; graphite powder(Superior Graphite, USA) was used as an active material for graphiteelectrodes. The electrolyte used in electrochemical cells was composedof 1M LiPF6 salt in a mixture of carbonates (Novolyte Technologies,USA).Characterization. Ellipsometry studies on swelling of polymer filmsdeposited on Si wafers in carbonates were performed using a COMPELautomatic ellipsometer (InOmTech Inc., USA) at an angle of incidence of70° Si wafers from the same batch were used as reference samples. Thethickness of the polymer binder was obtained by fitting theellipsometric data, assuming the refractive index of the binder andcarbonate to be 1.5. The mechanical properties of the polymer films (˜2micron) were measured with atomic force microscopy (AFM) by the tipindentation technique. Studies were performed using a Dimension 3100(Digital Instruments Inc., USA) microscope. Force-distance data werecollected using silicone cantilevers with spring constant of 40 N/m withapproaching-retracting probing frequency of 1-2 Hz. Force-volumemeasurements were used to obtain the stiffness distribution over thesurface of the sample. Measurements were performed on samples in both adry state and a “wet” state after the film was immersed in a 1:1:1mixture of dimethyl carbonate (DMC), ethylene carbonate (EC) and diethylcarbonate (DEC), similar to the electrolyte solvent used in theelectrochemical tests. PVDF (Kureha, Japan) in a dry state was used as areference and the stiffness data were normalized accordingly.

For NMR measurements sodium alginate was dissolved in D₂O andfreeze-dried twice to remove any exchangeable protons. Finalconcentration of the alginate for NMR measurements was 5 g/L. The 256spectra were collected on Bruker Avance 500 NMR spectrometer at 80° C.and averaged. HOD signal in a spectrum was suppressed with WATERGATEpulse sequence. Fourier Transform Infrared (FTIR) spectroscopymeasurements were performed using a Thermo-Nicolet (Thermo ElectronCorporation, USA) Magna 550 FTIR spectrometer equipped with aThermo-Nicolet Nic-Plan FTIR microscope. All samples were analyzed inthe attenuated total reflectance (ATR) mode using a Diamond ATRaccessory. For each spectrum 32 scans were collected at a resolution of4 cm⁻¹ from 4000 cm⁻¹ to 500 cm⁻¹. Background spectra were collected ina similar way. All the FTIR data were analyzed using a OMNIC E.S.Pversion 6.1a software (Thermo Scientific, USA). X-ray photoelectronspectroscopy (XPS) measurements were performed using a Thermo K-AlphaXPS system (Thermo Scientific, USA) equipped with a Al Kα radiation as asource, with an energy resolution of 1 eV for the survey scans and 0.1eV for high resolution scans of individual characteristic peaks. TheX-ray gun produced a 400 μm spot size, and an electron flood gun wasused to minimize charging. The system vacuum level was below 10-8 Torrduring the data collection. An emission angle of 90° was used.

SEM studies of the nanopowder and electrodes were carried out using aLEO 1530 SEM microscope (LEO, Japan, now Nano Technology SystemsDivision of Carl Zeiss SMT, USA). The in-lens secondary electrondetector was used for the studies, most of which were performed using anaccelerating voltage of 5 kV and a working distance of 2-5 mm. XRDstudies were performed using a PANalytical X'Pert PRO Alpha-1diffraction system (PANalytical, Netherlands) equipped with an incidentbeam monochromator. The system used only the Kα1 component of Curadiation, improving the overall quality of the collected powderdiffraction data. An accelerating voltage of 45 kV, current of 40 mA,2θ-step of 0.033° and a hold time of 79 sec was selected. The scan wascollected between 20 and 80 degrees. X'Pert HighScore Plus software(PANalytical, Netherlands) was used for spectral analysis. The nitrogenadsorption and desorption isotherms were collected at 77 K in the rangeof relative pressures of 0.001-0.99 P/P0 using TriStar II 3020 (V1.03)surface area and porosity measurement system (Micromeritics Inc., USA)and used for measurements of the specific surface area (SSA) and poresize distribution (PSD) in the 2-100 nm range. After drying the powderunder a vacuum at 80° C. for at least 12 h, 50-100 mg of each powdersample was degassed under a N₂ gas flow at 300° C. for at least 2 hprior to weighting and gas sorption measurements. For measuringelectrode porosity, no high temperature (300° C.) was used. The SSAswere calculated using the Brunauer-Emmett-Teller method usingMicromeritics DataMaster software. The relative pressure range of P/P0from 0.05 to 0.3 was used for multi-point BET calculations. Ultra highpurity gases (99.999%, Airgas, USA) were used for all experiments.

Electrochemistry. Working electrodes were prepared by casting a slurrycontaining an active material (either Si nanopowder mixed with carbonadditives or graphite) and a sodium alginate binder (15 wt. % for Sielectrodes and 10 wt. % for graphite electrodes) on a 18 μm Cu foil(Fukuda, Japan). Working electrodes consisting of active materials(either Si nanopowder mixed with carbon additives or graphite) and PVDF(9305, Kureha, Japan, 10 wt. % for graphite electrodes and 15 wt. % forSi electrodes) were used for the purpose of comparison. The activematerial in the Si electrodes contained 75 wt. % Si and 25 wt. % C. Theelectrodes were calendared and degassed in vacuum at 105° C. for atleast 4 hours inside an Ar-filled glove box (<1 ppm. of oxygen andwater, Innovative Technology, Inc., USA) and were not exposed to airprior to their assembly into cells.

Example #1: Assessment of Binder Performance Under Extreme Conditions

To assess binder performance under extreme conditions, charge-dischargecycling to nearly 100% depth-of-discharge (DoD) (to 0.01 mV vs. L/Li+)were performed without limiting the intercalation capacity. In contrastto studies on CMC binders, which often requires low Si (33 weightpercent (wt. %)) and high binder and carbon additive (33 wt. %/each)content, a high ratio of Si to C (Si:C=3:1) and a considerably smalleramount of binder (15 wt. %) were used for our tests. Alginate was usedas a binder in the experiments. The electrode slurries (water with asmall addition of (about 10 wt. %) alcohol was used as a solvent) werethoroughly mixed using an ultrasonic bath and a laboratory stirrer at600 rotations per minute (rpm) for at least 1 hour, cast on an 18micrometer (μm) Cu foil (Fukuda, Japan) using a 150 μm doctorblade,dried in air first at room temperature and then at 60 degrees Celsius (°C.) for at least 4 hours, degassed in vacuum at 100° C. for at least 2hours inside an Ar-filled glove box (<1 parts per million (ppm) ofoxygen and water, Innovative Technology, Inc., USA) and were not exposedto air prior to their assembly into the cells. The commercialelectrolyte was composed of 1M LiPF₆ salt in ethylene carbonate-diethylcarbonate-dimethyl carbonate mixture (EC:DEC:DMC=1:1:1 vol %) (NovolyteTechnologies, USA) with 5 wt. % addition of vinylene carbonate. Lithiummetal foil (0.9 mm thick, Alfa Aesar, USA) was used as a counterelectrode. 2016 stainless steel coin cells were used for electrochemicalmeasurements. The Cu current collector of the working electrode wasspot-welded to the coin cell for improved electrical contact. Charge anddischarge rates were calculated assuming the experimentally determinedcapacity for C and the maximum theoretical capacity for Si (4200 mAh/g),given the composition of the active material (either C or CSi mixture).Long-term cycling was performed in the 0.01-1 V vs. Li/Li⁺. Coulombicefficiency was calculated as 100% (C^(dealloy)/C^(alloy)), whereC^(dealloy) and C^(alloy) are the capacity of the anodes for Liinsertion and extraction. Arbin SB2000 (Arbin Instruments, USA), amultichannel potentiostat, was used for electrochemical measurements.For the purpose of illustrating the invention, there are shown andpresented exemplary embodiments of the invention; however, the inventionis not limited to the specific methods, compositions, electrodefabrication and devices disclosed. FIGS. 2A and 2B show theelectrochemical performance characteristics of the anode containingalginate. FIGS. 3, 4A, and 4B depict the scanning electron microscopy(SEM) image of the Si nanopowder used in some of the investigatedsamples, such as those presented in FIGS. 4A and 4B.

Si-based anodes prepared with the addition of alginate, as illustratedin FIGS. 4A and 4B demonstrated high capacity (near 3000 mAh/g at thefirst two cycles collected at slow (C/2) charge-discharge rate), veryhigh stability when tested at C/1 rate, and Coulombic efficiencyapproaching 100% with increasing cycle number.

The Si nanopowder had an average size below about 100 nanometers (nm).However, Si powder having different size distribution also yields goodperformance. Similarly, other anode materials are also expected to showsuperior performance if used with alginate as a binder or additive.

Example #2: Comparison of Na Alginate and PVDF Binders

The ratio of M-to-G monoblocks in alginates may range from 0.3 to 9,with a typical value in commercial samples being ˜1. Nuclear MagneticResonance (NMR) spectroscopy measurements, illustrated in FIG. 5, revealthat the ratio of M-to-G monoblocks in the Na alginate sample used forthe studies, described below, was 1.13. This ratio was calculated basedon integration of the peaks at 4.7, 5.3 and 5.7 ppm. Atomic forcemicroscopy (AFM) studies showed that in dry state films made of the Naalginate exhibit approximately 6.7 times higher stiffness than dry filmsof PVDF, shown in FIGS. 6 and 7. Interestingly, when immersed into anelectrolyte solvent the stiffness of alginate did not changesignificantly, shown in FIG. 8, while the PVDF films became nearly fiftytimes softer, shown in FIG. 9. Further, ellipsometry studies show nodetectable swelling of thin (˜70 nm) Na-alginate films in theelectrolyte solvent vapors. In contrast, PVDF films of similar thicknessattract significant amounts of carbonates from the vapor, demonstratingchanges in thicknesses of approximately 20%. The negligibly smallswellability of the alginate indicates a low level ofpolymer/electrolyte interaction. This property may prevent undesirableaccess of the electrolyte liquid to the binder/Si interface. The similarbehavior of Na-CMC binders likely explains their promising performancewith Si anodes as well. FIG. 23 A depicts the Young's modulus of Na-CMCin a dry state. FIG. 23B depicts the Young's modulus of Na-CMC in a wet(impregnated with electrolyte solvent) state.

Example #3: Silicon Nanoparticle Characteristics

Scanning electron microscopy (SEM) studies show the majority of Sinanoparticles used in our studies to be of elliptical or spherical shapewith diameter in the range of 20 to 100 nm, as illustrated in FIG. 10.Energy dispersive spectroscopy and X-ray diffraction, illustrated inFIG. 11, studies have reveal no impurities in the nanopowder. Theaverage Si particle size was calculated from the XRD data to beapproximately 37 nm. The shape of the N₂ adsorption/desorption isothermscollected on the Si nanopowder (Type II according to the Brunauerclassification) is typical for macroporous (>50 nm) solids withunrestricted multilayer adsorption, shown in FIG. 12. The specificsurface area (SSA) of the Si nanopowder calculated using theBrunauer-Emmett-Teller (BET) equation is 96 m²/g, which is much higherthan 0.5-10 m²/g found in graphites used in Li-ion batteries. Assumingthe density of Si nanoparticles to be 2.3 g/cm³, the average Si particlesize can be calculated to be approximately 27 nm, which is close to whatwe observed in SEM and estimated using XRD measurements. The electrodesprepared using Si nanopowder, conductive C additives and Na-alginateshow a uniform structure and a very smooth surface, illustrated in FIG.13, with small (<100 nm) pores visible between the nanoparticles. Theestimated electrode density is about 0.50 g/cm³. Assuming thetheoretical density of graphite, Si and alginate to be accurate, one canestimate the remaining pore volume of the electrode to be approximately5 times the volume of Si particles. In recent studies on electrochemicalalloying of Si in a nanoconfined pore space, the nano Si may undergo theirreversible shape changes upon the initial Li insertion, adapting tothe restricted shape of the rigid pore. In subsequent cycles, however,the Si—Li alloy may exhibit fully reversible shape changes. Therefore,even if the rigidity of the alginate binder would prevent electrodeexpansion upon the first Li insertion, the initial electrode porositycould provide space to accommodate the volume changes in Si duringcycling.

Example #4: Interactions of Na-Alginate with Si and C Particles

In order to evaluate the interactions of Na-alginate with Si and Cparticles electrodes consisting of pure Si/alginate and pure C/alginatemixtures were prepared. After drying the electrodes in vacuum (0.01Torr) at 105° C. for 4 hours, pieces of the electrodes were immersed inlarge beakers filled with de-ionized (DI) water (alginate solvent) andstirred for 4 hours. After filtering and drying in air, the Si (or C)particles were collected, immersed in DI water, stirred for 4 hours andfiltered. This procedure was repeated 5 times. Prior to spectroscopymeasurements, all samples were dried in vacuum at 105° C. for at least 8hours. The C_(1s) core-level x-ray photoelectron spectroscopy (XPS)spectra of the alginate and Si-alginate films show three characteristicpeaks corresponding to ether, alcoholic and carboxylate functionalgroups, shown in FIG. 14. As expected, the initial Si powder does notshow any signs of C atoms on the surface. Interestingly, in spite of theextensive purification of the Si powder after mixing with alginate, thepowder undoubtedly retains significant content of alginate residues onthe surface. A comparison of the C_(1s) spectra of DI water-cleaned Sinanopowder before and after mixing with Na-alginate suggests formationof strong hydrogen bonding between the hydroxylated Si surface andalginate carboxylic moieties. Somewhat similar conclusions could be madeby analyzing the Si_(2p) core-level peaks.

Prior to mixing with alginate, the Si nanopowder surface shows a strongbulk Si peak at approximately 99.2 eV and a peak corresponding tohydroxyl functional groups at approximately 103 eV, shown in FIG. 15.However, an additional peak corresponding to R(O)—O—Si at 103.9 eV isobserved after mixing Si with Na-alginate and vacuum annealing to forman electrode. This peak is mostly retained after the extensive cleaningof the Si nanopowder described above. Analogous XPS experiments with Cadditives suggest rather similar interactions between the polar groupsand defects on the carbon surface and alginate moieties.

Fourier transform infrared (FTIR) spectroscopy studies provide furthersupport for the strong bonding between the alginate and Si powder. ANa-alginate film exhibits a broad absorption band at about 3320 cm⁻¹related to hydrogen bonded O—H stretching vibrations, a peak at about1598 cm⁻¹ corresponding to O—C—O (carboxylate) asymmetric vibrations, apeak at about 1410 cm⁻¹ corresponding to O—C—O symmetric vibrations, apeak at about 1300 cm⁻¹ related to the C—C—H and O—C—H deformation ofpyranose rings, and a peak at about 1028 cm⁻¹ related to C—O—Casymmetric vibrations, among others, shown in FIG. 16. After electrodeformation the relative intensity of the 1300 cm⁻¹ peak related topyranose ring deformation vibrations decreases significantly whencompared to pure Na-alginate. This decrease provides another evidence ofa chemical interaction between the alginate and Si nanoparticles. Thestrong interactions between the binder and the Si surface have beenpreviously identified as one of the most critical factors affecting thestability of Si-based electrodes.

Coin cells with metallic Li counter electrode were employed to evaluatethe electrochemical performance of all of the electrodes. Since anassessment of the binder performance under extreme conditions was ofinterest, charge-discharge cycling to nearly 100% depth-of-discharge(DoD) (to 0.01 mV vs. L/Li+) were performed without limiting theintercalation capacity. In contrast to prior studies on CMC binders,which often required low Si (33 wt. %) and high binder and carbonadditive (33 wt. %/each) content, a high ratio of Si to C (Si:C=3:1) andconsiderably smaller amount of binder (15 wt. %) were used for testing.

Charge-discharge cycling performed with Li insertion capacity limited to1200 mAh per gram of Si showed stable anode performance for over 1300cycles (FIG. 25). In real-life applications, however, a noticeablevariation in the degree of lithiation of individual Si particles maytake place. Therefore, it is important to test the ability of Si anodesand Si-binder interface to withstand the largest volume changes takingplace during full lithiation. In our additional tests (FIGS. 17, 21, and24B-D, 25) we inserted Li to nearly 100% depth-of-discharge (DoD)- to0.01 mV vs. L/Li⁺ and additionally held the anode at this potential forover 10 min. Since the average time of full Li insertion into 100 nmdiameter Si nanoparticles is 6 minutes and average Si particles in ourelectrode are only 27 nm, this procedure warranted that a large portionof the Si particles (close to a Cu foil) was fully lithiated. In spiteof the severe testing conditions an alginate binder allowed for a stableperformance of Si electrodes (FIGS. 17, 21, 24B, 25). This is incontrast to Si anodes with PVDF and Na-CMC, which demonstrated poorstability (FIG. 26).

The reversible deintercalation (Li-extraction) specific capacity of theSi anode reached 3040 mAh/g at a current density of 140 mA/g, shown inFIG. 17, which is over eight times higher than the theoretical specificcapacity of graphite (372 mAh/g). The contribution of Si nanopowderalone could be calculated as about 4000 mAh/g, which is consistent withobservations on other nanoSi materials, but is noticeably higher thanwhat was previously observed for microSi. The contribution of the Cadditives used in our electrodes was estimated about 160 mAh/g (40mAh/0.25 g). The volumetric anode capacity was determined to be about1520 mAh/cm³ at 140 mA/g current density, which is 2.5 times higher thanabout 620 mAh/cm³ for graphitic anodes. At a slightly higher currentdensity of 280 mA/g, the anode capacity remains high at 2910 mAh/g, butonce the current density is significantly increased to 4200 mA/g thecapacity decreases and equilibrates at 1700 mAh/g. This reduced capacityis still about 4.5 times higher than the theoretical capacity ofgraphite and is about 9-to-20 times higher than the experimentallydetermined capacity of graphites (85-190 mAh/g) at such a high currentdensity. In spite of the very severe testing conditions, the electrodedemonstrated stable performance for over 300 cycles. The combination ofan ultra-high reversible specific capacity and the demonstratedlong-term stability achieved in comparable electrochemical tests isunprecedented not only for Si particles but also for any othercompetitive technologies. FIG. 25 depicts the electrochemicalperformance of the alginate-based nanoSi electrodes (electrodedensity=0.50 g cm-3, weight ratio of Si:C=3:1). Specifically, in FIG. 25reversible Li deintercalation capacity and coulombic efficiency of thenanoSi electrodes with an alginate binder vs. cycle number for Liinsertion level fixed to 1200 mAh gSi⁻¹ is shown.

A stable binder for Si anodes needs to posses several criticalproperties. First, a very weak binder-electrolyte interaction is neededfor the long-term anode stability. Indeed, binders that provide at leastsatisfactory performance in Si anodes (including CMC and PAA) experiencevirtually no swelling in commonly used electrolytes. Once a solventreaches the Si electrode surface by permeating through a binder layer,it decomposes. The solvent decomposition products deposited in theregion between the binder and the Si would significantly weaken theSi-binder bond strength. Therefore, little-to-no interactions betweenthe binder and the solvent are critically needed to prevent access ofthe solvent molecules to the Si-binder boundary.

Another critical property of an ideal Si binder is to allow the accessof Li ions to the Si surface. Therefore, if a binder (such as alginate,CMC, PAA and others) is not permeable to solvent molecules, it shouldeither cover only a portion of the Si surface or remain permeable to Liions. Due to the small size of Si nanopowder and its resultant highsurface curvature, the number of anchor points between a binderpolymeric chains and Si particles is limited, suggesting that a portionof the Si surface should indeed be directly exposed to the electrolyte.XPS studies on alginate-coated Si particles indeed show that a portionof Si surface is alginate-free. To identify the conductivity Li-ionsthrough Na-alginate a thin (1 μm) layer was deposited on Cu foil andperformed cyclic voltammetry and electrochemical impedance spectroscopytests with Li foil as a counter electrode. Both tests revealed small,but sufficient ionic conductance. From the impedance data the Warburgconstant was determined and the diffusion coefficient of Li inNa-alginate was estimated to be about 10⁻⁸ S·cm⁻¹. While this is fourorders of magnitude smaller than the diffusion coefficient of Li insolid electrolytes, the nm-level thickness of a Na-alginate layercompensates for its limited diffusivity. The proposed mechanism of iontransport through the alginate is via hopping of Li ions between theadjacent carboxylic cites, similarly to alginate's function for the iontransport in the algae cells.

Third, an ideal binder should assist in building a deformable and stablesolid-electrolyte interphase (SEI) on the Si surface. High CoulombicEfficiency (CE) is important for practical applications and ischallenging to achieve in Si-based anodes due to the need to maintain astable SEI layer, in spite of the large changes in particle volume (andtherefore surface area) during the battery operation. In ultra-thin Sifilms high stability is achieved because the film surface area does notchange during cycling and the volume changes are accommodated largelyvia variation in film thickness. Thus maintaining a stable SEI is not achallenge. In thicker films that exhibit cracks at the currentcollector-Si interface and thus experience some surface area changes,electrolyte additives are needed to achieve a stable SEI. Infree-standing Si nanowires that do not need a binder, but experiencemuch more significant surface area changes upon cycling, the unprotectedSi fails to maintain a stable SEI, causing continuous Li consumption,increasing Si surface roughness and decreasing CE with every cycle. Sinanowires commonly demonstrate CE of only 93-97%. In contrast,electrodes described herein show improving CE with every cycle, asillustrated in FIG. 17, suggesting that the alginate binder contributesto building a stable passivating SEI layer. The alginate-basedelectrodes demonstrate an average CE of 98.5% for the first 100 cyclesand an average CE of 99.9% for cycles 101-300. In order to test thehypothesis that Na-alginate assists in building a stable SEI XPS studieson the electrodes before and after cycling were performed, and theresults are illustrated in FIGS. 18A-C. Indeed, the surface chemistry ofthe SEI did not noticeably change between the 10^(th) and 200^(th)cycle, suggesting excellent SEI stability and fully supporting ourhypothesis. A similar positive impact on improving CE was also observedwith a CMC binder, but only when its relative amount is several timeshigher.

Nonetheless, even if the stability of the SEI and binder-Si interface isachieved, binders that show virtually no extensibility (CMC, PAA andalginate) require the Si electrode to possess sufficient pore volume,needed for Si expansion. Indeed, increasing the pore volume of CMC-basedSi electrodes significantly improved their stability. The lack ofsufficient pore volume may cause sealing of the inter-particle pores(and thus a dramatic reduction in the ion transport) and mechanicalfailure of the electrode during operation. The smallest sufficient porevolume should be larger than the total volume of Si expansion forseveral reasons. First, the shape of the Si particles and the shape ofthe pores are different. Therefore, at the fully expanded state somepore volume will remain unfilled. Note that plastic deformation oflithiated Si nanoparticles may take place, thereby reducing thestrictness of the requirements on local pore shape and size. However,strong bonding of the binder to Si particles and high binder stiffnessis needed because the endurance limit of the binder and the binder-Siinterface must exceed the internal stresses in the electrode caused bythe volume expansion of Si nanoparticles. Second, the SEI formationrequires some available volume as well. Furthermore, at the expandedstate Si particles could be pressed against each other, inducing highlyundesirable damage in the SEI. Finally, open pores not filled with anyelectrolyte decomposition products are needed for the rapid transport ofLi ions within the electrode. Too large a pore volume, however, willlead to a decrease in the volumetric capacity of the anode. Since theconsiderations discussed above make it difficult to precisely predictthe minimum pore volume, additional experiments to intentionally densifythe electrodes were performed. When the electrode density was increasedto ˜0.75 g/cc (the total pore volume equal to 2.7 time the volumeoccupied by Si particles), the electrodes showed noticeably worseperformance. Therefore, it is estimated that the ideal pore volume issomewhere between 3 and 6 the volume of Si component of the electrode,provided the binder has properties similar to that of alginate or CMC.

Since the mechanical properties of Na-CMC and Na-alginate are similar,and since both binders do not interact with electrolytes, the dramaticdifference in their performance in Si electrodes of similar porositylevels is most likely linked to the concentration of functional groupsbonded to Si surface. In alginate carboxylic groups are naturallypresent and evenly distributed in the polymer chain, while in CMC theyare synthetically induced and their distribution is random, where somemonomeric units may have more than one carboxylic group, and other havenone. The higher concentration and a more uniform distribution of thecarboxylic groups along the chain in alginate could be responsible forthe better transport of Li ions in vicinity of Si particles, moreuniform coverage and more efficient assistance in the formation of astable SEI layer on the Si surface (FIGS. 18A-18C). Alginatemacromolecules are also much more polar than the CMC polymer chains,which can ensure better interfacial interaction between the polymerbinder and the particles and stronger adhesion between the electrodelayer and Cu substrate. This large difference in chemistry of CMC andalginate results in major differences in their behavior. For example,the alginate solution in water has dramatically higher viscosity thanCMC (FIG. 1A). This high viscosity prevents Si particles fromsedimentation and aggregation during the electrode formation, as wateris evaporating, resulting in high slurry uniformity. This uniformity isknown to be critical for obtaining uniform distribution of activematerials within the anode needed for the long-term electrode stability.Alginate solution also exhibits much higher degree of shear-thinningbehavior (FIGS. 1B, 1C), which offers an opportunity to lower a slurryviscosity needed for fast homogenization by increasing the mixing rateand an opportunity to increase a slurry viscosity for porosity anduniformity control during the electrode formation by lowering the mixingrate. To achieve viscosity comparable to alginate solutions,significantly higher CMC content is needed. Similarly, in order to get aremotely comparable performance with a CMC binder one needs to increasethe binder:Si ratio by a factor of four. The high binder contentdecreases the electrical conductivity of the electrode and necessitatesthe utilization of a higher content of the conductive carbon additives(increasing the C:Si ratio by the factor of three), which inevitablylowers the electrode specific capacity.

To further characterize the behavior of the alginate-based electrodecyclic voltammetry experiments were performed. The differential capacitycurves show one broad Li insertion (cathodic) peak at about 0.21 V andtwo Li extraction (anodic) peaks at 0.33 V and 0.51 V, shown in FIG. 19,all commonly observed in Si anodes. The origin of the potentialdifference between the corresponding peaks in the cathodic and anodicdirections has been the subject of recent discussions and is commonlymodeled by a thermodynamic (rate independent) hysteresis. The physicsbehind the hysteresis is not yet well understood. The first 0.33 Vanodic peak is not always observed. In some Si—C nanocompositeparticles, for example, only one Li extraction peak at about 0.5 Vappears. Therefore, the 0.33 V peak could be related to the surfaceproperties of Si. A small Li extraction peak observed at about 0.17 Vcorresponds to Li deintercalation from C additives. The five cyclicvoltammetry cycles demonstrate high reproducibility, indicative of goodanode stability.

FIGS. 20 and 21 show the shape of the galvanostatic Li insertion andextraction profiles for the produced Si anodes. The shapes of theprofiles are similar to the profiles previously reported in literaturefor other Si electrodes. In contrast to intercalation-type electrodematerials, these profiles do not exhibit strictly horizontal plateausand cover a larger potential range. Interestingly, the Li extractionprofiles become more horizontal and exhibit slightly smalleroverpotential with cycling, suggesting a gradual improvement in thedischarge kinetics. The current-dependent overpotential increases the Liextraction potential when current density is increased from 140 to 4200mA/g. By comparing the Li extraction capacities achieved at differentcurrent densities, we can conclude that these electrodes possess onlymoderate rate capability, inferior to that achieved in Si—C compositeanodes with hierarchical porosity or in Si nanowires and much inferiorto what can be achieved in supercapacitors. The advantage of thistraditional battery technology, however, is higher volumetric capacity,higher CE and compatibility with existing manufacturing techniques.Further electrode optimization and introduction of additional pores isexpected to significantly increase the rate performance, because thediffusion of Li into/out of Si nanoparticles can be achieved withinminutes.

In addition to improving the stability and CE of Si anodes, the alginateproperties provide advantages to other electrodes, such as traditionalgraphitic anodes. FIG. 22 compares the first charge-discharge profilesof graphitic anodes made with a traditional PVDF binder and with aNa-alginate one. Replacing PVDF with lower cost,environmentally-friendly alginate not only improves the anode dischargecapacity, but also increases the first cycle CE from 88 to 94%. Thestronger bonding to graphite likely allowed the alginate to achieve ahigher electrochemical utilization of the active material. Commercialanodes demonstrate cycle life up to ˜5,000 cycles, and their degradationis caused by the deterioration of the SEI layer and electrode integritydue to small volume changes in the graphite during cycling. It might beexpected that the stronger bonding, higher stiffness and the presence ofhigh concentration of precisely positioned carboxylate functional groupsin alginate may similarly improve the SEI stability and cycle life ofgraphite and other anode materials while simultaneously lowering theanode production cost and improving the environmental friendliness ofthe overall fabrication process, providing an immediate impact to oursociety.

Alginate, an attractive abundant low-cost environmentally friendlyrenewable material produced by photosynthesis in algae, offersoutstanding performance as a binder in battery electrodes, unmatched bythe current technology, as well as a separator material and/or a coatingat the surface of the electrode.

While the present disclosure has been described in connection with aplurality of exemplary aspects, it is understood that other similaraspects can be used or modifications and additions can be made to thedescribed aspects for performing the same function of the presentdisclosure without deviating therefrom. Therefore, the presentdisclosure should not be limited to any single aspect, but ratherconstrued in breadth and scope in accordance with the appended claims.

What is claimed is:
 1. A battery electrode comprising: a conductivemetal substrate; and an electrode active material comprising activeparticles, wherein each of the active particles is a silicon-carboncomposite, dispersed in a binder coupled to the conductive metalsubstrate, wherein the binder comprises an alginate material, whereinthe electrode active material is substantially homogenously dispersed inthe alginate material.
 2. The electrode of claim 1, wherein theelectrode active material further comprises a conductive carbon additivemixed in with the active particles.
 3. The electrode of claim 1, whereinthe silicon-carbon composites have a three-dimensional dendriticparticle structure.
 4. The electrode of claim 1, wherein the alginatematerial comprises a salt of alginic acid.
 5. The electrode of claim 4,wherein the salt of alginic acid comprises an alkaline salt of alginicacid or an alkaline earth salt of alginic acid.
 6. The electrode ofclaim 1, wherein the binder further comprises a polymer.
 7. Theelectrode of claim 1, wherein the binder further comprises a polymerblended with the alginate material.
 8. The electrode of claim 1, whereinthe binder further comprises a polymer grafted with or cross-linked withthe alginate material.